74 ETHEL NICHOLSON BROWNE 



or very nearly the same number of chromosomes. This fact has 

 led Montgomery and McClung to the view that the number and 

 arrangement of chromosomes should be considered as an important 

 character in taxonomy. More recently, McClung ('08) has 

 expressed this view forcibly in his paper on ''Cytology and Tax- 

 onomy." It is of interest in this connection to find that in the 

 Oenotheras, according to Lutz ('12), all individuals having a given 

 type of vegetative character have the same number of somatic 

 chromosomes, irrespective of the origin of these individuals, 

 whether hybrid or mutant. 



That one method by which a change in the chromosome num- 

 ber has taken place is by the fusion or separation of particular 

 chromosomes seems highly probable from the evidence given by 

 Notonecta, where we have all the stages in the process in the 

 three species. Such a process may also be indicated by the d- 

 chromosome in Nezara (Wilson '11). A somewhat similar idea 

 was put forth by Montgomery COl) before the relation of the 

 A"-chromosome with sex had been established, to explain the 

 occurrence of an odd number of chromosomes in the spermato- 

 gonia! groups of some of the Hemiptera; the odd number represent- 

 ing, he believed, a transition stage between two even numbers. 

 A change in number by a process of fusion has been advocated 

 by AlcClung ('05) in regard to the multiple chromosomes in the 

 Orthoptera. A change in number by a process of splitting has 

 been advocated by Payne ('09) in the case of the multiple X-ele- 

 ment in the reduvioids, and this may likewise apply to that 

 of many other forms, such as Phylloxera, Syromastes or Ascaris 

 lumbricoides, as has been indicated by Wilson ('11). A second 

 probable method of change is by a process of progressive reduc- 

 tion and final disappearance of particular chromosomes, as was 

 originally suggested by Paulmier ('99) in the case of the small 

 m-chromosomes of the Coreidae, and later by Wilson in the case 

 of the F-chromosome. 



These two methods will account for gradual and slight changes 

 in the chromosome number. Such wide variations as occur in 

 closely related species, e.g., in Banasa, Thyanta, and the phyl- 

 loxerans, must be accounted for in some other way. Wilson ('11) 



